Secret agents specialising in the “dark arts” may have tried to cover up the mysterious death of an MI6 spy found locked in a holdall, a coroner has heard.

The family of Gareth Wyn Williams believe a third party was present when the spook died, or broke into his home to destroy evidence.

The allegations came as it emerged blundering police chased a key line of inquiry for 18 months before realising the ‘evidence’ was actually a laboratory DNA mix-up.

At a pre-inquest review into the 31-year-old’s death, lawyer Anthony O’Toole said it must be established why there was no evidence of another person in his London apartment when he died.

He said: "The impression of the family is that the unknown third party was a member of some agency specialising in the dark arts of the secret services - or evidence has been removed post-mortem by experts in the dark arts."

Mystery: The body of Gareth Wyn Williams is removed from his flat in Pimlico in August 2010 (Image: Daily Mirror)

Mr O'Toole said Mr Williams "could have been actively deployed" as an agent up to five months before his death.

"In our submission, to properly explore the circumstances of the death, we need to establish the deceased's work," the lawyer said.

Mr O'Toole said relatives wanted to know why the alarm was not raised when Mr Williams initially failed to turn up to work.

By the time officers arrived at his flat, his body was so decomposed that evidence had been lost.

Fresh questions over the cause of his death were raised after it emerged that two areas of investigation were red herrings.

Red faced Scotland Yard has been forced to admit that a spot of DNA found on the 31-year-old spook’s hand in 2010 actually belonged to a scientist at the crime scene.

After 18 months of chasing the lead, the vital mistake only came to light two weeks ago.

The force revealed that a Mediterranean couple police wanted to speak to were irrelevant to Mr Wyn Williams' death.

Unconnected: The Mediterranean couple wanted by police were in fact nothing to do with the spy's death (Image: Getty)

The naked and decomposing body of the top codebreaker was found in the bathroom of his flat in Pimlico, central London, in August 2010.

The mathematics prodigy was found locked in a North Face holdall in the bathtub at the top floor flat in Alderney Street.

A battery of post-mortem tests failed to determine how he died and police originally found it would have been impossible for him to have locked himself inside.

The mathematics genius worked as a cipher and codes expert for GCHQ, the Government listening station, but had been on secondment to the Secret Intelligence Service (MI6).